The present invention relates to a method of controlling an automotive electric machine and in particular to a method of compensating for a machine to machine variance of magnetic flux.
Torque produced by a synchronous permanent magnet electric machine, such as is used in an automotive vehicle for a traction motor, is dependent on the magnitude of back electromotive force (EMF) of the machine. Variation of the back EMF of the electric machine causes a corresponding variation in the torque output of the machine. Control programming developed for the machine may compensate for at least some of the variation of the back EMF. Typically, the programming compensation is calibrated using a prototype electric machine.
However, the compensation program may not account for back EMF differences due to a machine specific variance from the prototype machine to a production machine. Possible sources of the variance in the production machine includes air gap tolerances, magnet skew, rotor iron, stator winding, magnetization process used, magnet variation, and supporting structure variation, all of which may be compounded when components of the machine are provided by different suppliers. The compensation program may not account for the back EMF from the machine specific variance because the variance is machine and installation specific, which makes the variance difficult to predict before assembling and rotating the machines.